Method and circuit for processing sensory input signals of the type obtained from coriolis mass flow rate sensors and the like

ABSTRACT

An improved method and circuit for processing sensory input signals of the type obtained from Coriolis mass flow rate sensors and the like including differencing circuitry responsive to inputs received from two motion pick-up devices and operative to generate a &#34;different&#34; signal which is directly proportional to the difference therebetween, integrator circuitry for reducing the frequency dependency by one order and shifting the phase of the difference signal by 90 degrees, summing circuitry responsive to the signals generated by the two pick-ups and operative to develop a &#34;sum&#34; signal which is proportional to the sum thereof, and dividing circuitry for dividing the integrated difference signal by the sum signal to develop an output signal which is directly proportional to the rate of mass flowing through a conduit to which the two pick-up devices are attached.

The present invention relates generally to Coriolis type mass flowmeters and more particularly to an improved method and circuit forprocessing two electrical signals obtained from motion detectors coupledto one or more tubular conduits carrying a flowing mass and driven orexcited so as to experience Coriolis acceleration.

DESCRIPTION OF THE PRIOR ART

Numerous techniques are known in the prior art for processing theinformation that can be obtained by measuring the Coriolis forces or theeffects therefrom induced in a straight, bent or looped conduit. Forexample, in the Sipin U.S. Pat. Nos. 3,329,019, 3,355,944 and 3,485,098the velocity of conduit displacement on opposite sides of a "drive"point is measured and the difference there between is read using an ACvoltmeter calibrated to provide an indication of mass flow rate.However, as will be pointed out below, the use of the difference betweentwo velocity signals as a means of indicating mass flow rate involves afactor of w corresponding to the drive frequency. If the drive frequencyis not constant, errors in determining mass flow rate result. Also,under resonant operation, the drive frequency, w varies with fluiddensity, and such variations result in substantial mass flow ratemeasurement error. Sipin did not recognize this source of error.

Sipin also controlled his driving means with a signal that wasproportional to the sum of the two "velocity" signals measured. However,it has been found in the use of similar circuits that, as circuitcomponents age, or if such components are replaced, or the conduitdriving means (e.g. magnet and/or coil) are replaced, the sum of thevelocity signals will not provide for constant drive level. Thus, errorsin the mass flow rate measurement will result in direct proportion tochanges in drive amplitude, and flow meter recalibration will benecessary.

Dahlin et al, U.S. Pat. No. 4,660,421 and Kappelt, U.S. Pat. No.4,655,089, teach the detection of non-linear variation of phase shiftwith mass flow rate, a technique which although effective, isundesirable from a signal processing view point. Kappelt also explainshow the time difference method disclosed in the Smith Pat. Nos.RE31,450, U.S. Pat. Nos. 4,422,338 and 4,491,025, fails to properly takeinto account problems associated with drive frequency variations. Eachof the systems disclosed in the above mentioned patents can be adverselyaffected by excessive vibrational noise or hydraulic noise creatingspurious signals in the motion detectors, and such noise will exciteunwanted resonant vibratory modes in the tubular conduits whichcontribute adversely to signal measurement.

In an attempt to provide an accurate measurement at or near theoscillation midplane, the Smith U.S. Pat. No. 4,422,338 requires the useof analog sensors which are linearly representative of the actual motionover the full range of motion, a limitation that adds both cost andcomplexity to the design.

Roth U.S. Pat. Nos. 3,132,512 and 3,276,257 disclose methods ofprocessing analog signals obtained from "velocity" pick-up devices, butrequire additional signal inputs obtained from a driving motor, or othervelocity pick-ups, to provide reference signals for his signalprocessing. Besides requiring a pair of sensing coils and a pair ofreference coils, Roth connects his sensory coils in series opposingfashion, a technique that requires precise matching of sensing pick-upsto obtain no flow rate signal at zero flow.

SUMMARY OF THE PRESENT INVENTION

It is therefore an object of the present invention to provide a novelmethod and circuit for measuring mass flow rate using informationcontained from two sensors affixed to a vibrated conduit and which hasno frequency dependency and no adverse dependency on fluid density.

Another object of the present invention is to provide a method andcircuit of the type described which will not have its calibrationadversely affected by changes in drive level caused by aging, driftingcomponent parameters.

Still another object of the present invention is to provide a method andcircuit of the type described in which a continuous output signal isprovided that varies linearly with mass flow rate.

Yet another object of the present invention is to provide a method andcircuit of the type described which is not adversely affected byexternal mechanical or internal hydraulic noise or resonant modes ofoperation at frequencies in proximity to the operating frequency of thedevice.

A still further object of the present invention is to provide a methodand circuit of the type described which does not require motion pickupdevices that respond linearly throughout their entire range ofmeasurement.

Still another object of the present invention is to provide a method andcircuit of the type described which does not require the generation ofadditional signals to provide reference inputs.

Briefly, a preferred embodiment of the present invention includesdifferencing circuitry responsive to inputs received from two motionpick-up devices and operative to generate a "difference" signal which isdirectly proportional to the difference therebetween, integratorcircuitry for reducing the frequency dependency by one order andshifting the phase of the difference signal by 90 degrees, summingcircuitry responsive to the signals generated by the two pick-ups andoperative to develop a "sum" signal which is proportional to the sumthereof, and dividing circuitry for dividing the integrated differencesignal by the sum signal to develop an output signal which is directlyproportional to the rate of mass flowing through a conduit to which thetwo pick-up devices are attached.

In accordance with the present invention, "velocity measurements" areutilized to provide highly accurate mass flow rate measurements. This isin contrast to the teaching of Smith '450 wherein it is indicated that"velocity measurements" provide "at best only minimal results". Morespecifically, in distinguishing Sipin '098 the Smith '450 patent teachesthat the disclosed method and apparatus "is specifically structured tominimize or obviate the forces generated by the two non-measuredopposing forces, i.e., velocity drag and acceleration of mass. Thiseffort has been successful to the point where such forces are present incumulative quantities of less than 0.2% of the torsional spring force."In contrast to Smith, the present invention provides a way of measuringmass flow rate even in the presence of substantial competing forces. Forexample, the driving oscillation of the conduit introduces inertialaccelerations that vary from zero (when the conduit(s) is at its restposition) to values that exceed the magnitude of the mass flow inducedCoriolis forces by many orders of magnitude when the conduit is at itsextremes of motion. Additionally, helical conduits such as thosedisclosed in the Dahlin et al '421 patent are subjected to additionalforces resulting from fluid hydrostatic pressure and centrifugal forcesthat can cause the helical conduit to experience forces causing it to"twist" or distort. Such forces would adversely impair the accuracy ofmass flow measurement if such a conduit, or other conduit of athree-dimensional nature, were employed within the context and scope ofthe teaching of the '450 patent.

In further contrast to the Smith '450 patent, the present inventionprovides a new way of measuring mass flow rate throughout theoscillation cycle of a vibrating conduit without the limitation ofhaving to minimize the Smith's "non-measured opposing forces".

An important advantage of the present invention is that a signalprocessing circuit is provided wherein the response characteristics ofthe two pick-up devices need not be precisely matched in harmoniccontent.

Another advantage of the present invention is that it provides a circuithaving the ability to reject extraneous electrical noise introduced intothe motion pick-ups from electrical, mechanical and/or hydraulic noisesources.

These and other objects and advantages of the present invention will nodoubt become apparent to those skilled in the art after having read thefollowing detail description of the preferred embodiments which areillustrated in the several figures of the drawing.

IN THE DRAWING

FIG. 1 is a diagram schematically illustrating the principal operativecomponents of a Coriolis mass flow meter;

FIG. 2 is a block diagram generally illustrating the circuit andprocessing method of the present invention;

FIG. 3 is a block diagram schematically illustrating a preferredembodiment of a signal processing method and apparatus in accordancewith the present invention:

FIG. 4 is a diagram schematically illustrating the functional componentsof a lock-in amplifier(L.I.A.) of the type used in the embodiment ofFIG. 3;

FIG. 5 is a diagram illustrating a portion of the circuit of FIG. 3wherein a differentiator of the sum signal is substituted for theintegrator of the difference signal. The L.I.A. outputs will be the sameas FIG. 3;

FIGS. 6, 7 and 8 illustrate alternative circuit arrangements foraccomplishing the signal division function shown in FIG. 2;

FIG. 9 is a block diagram illustrating an alternative embodiment of aportion of the present invention; and

FIG. 10 is a block diagram illustrating an alternative embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1 of the drawing, there is shown, in simplifiedform, a Coriolis mass flow rate measurement device including a sensortube 10 which can be of any configuration, but for purposes ofillustration is shown as a cross-over loop of the type disclosed in U.S.Pat. No. 4,660,421. It is to be understood however, that any tubeconfiguration can be utilized. As illustrated, the sensor 10 includes anelongated, generally helically wound cross-over loop 12, each end ofwhich is suitably affixed to a mounting, as indicated at 14 and 16respectively. Coupled to the drive portion 18 of the loop 12 is amagnetic drive source D for exciting the loop and causing it to vibrateup and down as depicted by the arrow 20.

Coupled to the leftmost loop extremity 22 and rightmost loop extremity24 are magnetic pickup devices P1 and P2 respectively, which, in thepreferred embodiment, are "velocity-type" pick-ups. It will beappreciated however, that any other suitable type of pickup could beutilized, such as those of the "position" or "acceleration" type, forexample. Additionally, it will be appreciated that pick-up devices P1and P2 need not be located at opposing or "symmetrically opposite"positions on the conduit.

Drive energy for unit D, typically in the form of an alternating voltageis provided by an electronic drive control and monitoring system 26which also receives signals generated by the pick-ups P1 and P2, and inresponse thereto develops mass flow rate information for recording ordisplay. As is well known in the prior art, in response to theoscillatory forces applied to of the loop 12 by drive unit D,corresponding motion will be transmitted to the loop extremities 22 and24, and such motion will be detected by the pick-up units P1 and P2. Asis also well known, with mass flowing through the loop 12, signalsgenerated by the respective pick-up units will differ by an amount whichis related to the mass flow rate of material passing through the loop12.

As indicated above, it has historically been the practice to measureeither time difference of passage of the tube motion sensing pointsthrough a mid-plane of oscillation, as disclosed in the Smith '450, '338or '025 patents; or to measure the phase shift between the two sensedsignals, as disclosed in Dahlin et al '421 patent; or to measure thevoltage difference between the outputs of the two sensors as a measureof mass flow rate when the input to drive D is held constant, as taughtin the Sipin '019, '944 and '098 patents. However, as pointed out above,such measurement techniques are all subject to certain disadvantages.

In FIG. 2 of the drawing, a novel signal processing circuit and methodin accordance with the present invention is illustrated generally inblock diagram form, and includes a differencing component 30, a summingcomponent 32, an integrating component 34, and a dividing component 36.As suggested by the drawing, the differencing component 30 responds tothe voltage signals V1 and V2 generated by pick-up units P1 and P2, anddevelops a difference signal

    V3=V1-V2.                                                  (1)

The difference signal V3 is then integrated over a period of time by theintegrating component 34 to develop a voltage

    V5=∫.sup.t (V1-V2)dt                                  (2)

At the same time, summing component 32 responds to the voltages V1 andV2 and develops a sum voltage

    V4=V1+V2                                                   (3)

The voltages V4 and V5 are then input to the dividing component 36 thatin turn develops an output voltage

    V6=V5/V4                                                   (4)

which is directly proportional to mass flow rate. However, as contrastedwith prior art techniques, such signal is not subject to error caused bydisturbances of the type discussed above.

As an illustration of the present invention, consider the following:under no-flow conditions and with a drive excitation voltageproportional to Ae sinwt where

w=the driving or excitation frequency applied to the drive "point" D,and

Ae=the "excitation" or drive amplitude at the location of the pickups P1and P2.

The voltage signals generated by pick-ups of the velocity types, P1 andP2, will be equal to each other as represented by

    V1=V2=Ae w coswt                                           (5)

However, when mass is flowing through the tube 10, it can be shown that

    V1=w[(Ae coswt)-(Ar sinwt)]                                (6)

and

V2=w[(Ae coswt)+Ar sinwt)] (7)

where

Ar=the "response" amplitude (i.e., the "Coriolis" amplitude) at thelocation of pickups P1 and P2.

Assuming the tubular conduit structure obeys Hooke's Law in response toCoriolis forces, it can also be shown that

    Ar=k Ae w Qm

and therefore,

    Qm=Ar/k Ae w                                               (8)

where k is a proportionality factor (that can be temperature dependent),and Qm is the mass flow rate to be determined.

Now, from equations (6) and (7), V3 and V4 can be defined as

    V3=V1-V2=-2 w Ar sinwt                                     (9)

and

    V4=V1+V2=2 w Ae coswt,                                     (10)

and letting

    V5=∫.sup.t (V1-V2)dt=2Ar coswt                        (11)

and

    V6=V5/V4=Ar/w Ae,                                          (12)

it can be seen that V6 is directly related to Qm, That is,

    Qm=(1/k) V6.                                               (13)

It will thus be appreciated that the system represented by the blockdiagram of FIG. 2 is purely analog in nature and involves no timingmeasurements.

The heart of the present invention begins with the use of the differencesignal V3=V1-V2.

Although the nature, character and information content of the signal V3is completely analogous to the output of Sipin's difference amplifierdesignated 92 in FIG. 10 of Sipin's '944 and '098 patents, theadditional processing performed by the present invention on thisdifference signal, i.e., the integration and division of the integratedresult by the sum of V1 and V2 constitutes a substantial improvementover the Sipin method by eliminating the otherwise adverse effects thatfluid density variations and drive amplitude and frequency variationshave on the accuracy with which the mass flow rate Qm can be determined.

In FIG. 3, a block diagram more specifically illustrating an actualimplementation of the present invention is likewise depicted insimplified form. In this embodiment the outputs of the two velocitypick-ups labeled P1 and P2 are amplified by gain stages designated U1and U2. As suggested by the variable resistor R, the gain factor of eachinput channel is adjusted to compensate for unequal detected velocitysignals, so that the amplifier outputs have approximately equalamplitudes. These two signals are then presented both to a differenceamplifier U3 and to a sum amplifier U4. Although there may be some gainassociated with the difference amplifier U3 and the sum amplifier U4,the difference signal output by U3 is proportional to the differencebetween the two input voltages, and the sum signal output by U4 isproportional to the sum of the two input voltages.

The difference signal is then further processed by integration using anintegrating amplifier designated U5, and the integrated differencesignal is presented to a first lock-in amplifier U7. In the sum channel,the sum signal output by U4 is presented to a second lock-in amplifierU8. The two lock-in amplifiers (U7 and U8) share a common referencesignal derived by taking the output of the summing amplifier U4 andinputting it into a 0-Volt biased comparator U6 to obtain a square wave,the frequency of which is the same of that of the sum signal. The squarewave is then used as a reference input for both lock-in amplifiers. Itshould perhaps be pointed out that although the preferred embodimentincludes two lock-in amplifiers U7 and U8, the amplifier U8 could bereplaced by a precision rectifier or peak detector or the like.

One way of viewing the operation of a lock-in amplifier is that it islike a very narrow band, frequency selective rectifier. Any frequenciespresent in the signal other than the frequency of the reference, such asadditional signals due to external mechanical noise or hydraulicpulsations in the meter that would normally cause extraneous signals tobe present in the motion sensing pick-ups P1 and P2, will be rejected bythe lock-in amplifiers. The signals output by the lock-in amplifiers areDC voltages that are further amplified by the buffer stages U9 and U10and then converted to proportional frequencies by voltage-to-frequencyconverters designated U11 in the difference channel and U12 in the sumchannel.

Counters U13 and U14 are used to accumulate counts of each cycle outputfrom the voltage-to-frequency converters U11 and U12, and the counteroutputs are interfaced to a micro-processor MP which effectively dividesthe contents in counter U13 by the contents in counter U14. This is tosay that the number N1 stored in counter U13 is divided by the number N2stored in counter U14, and this ratio is related linearly to mass flowrate. The output of processor MP is then input to a suitable readoutdevice R which provides an output signal that is presented as either acurrent, a voltage, a frequency, or a readout on a visible display.

In FIG. 4 a schematic illustration is provided of the principaloperative components of a lock-in amplifier as an aid to understandingcertain benefits achieved through the use of such amplifier in thepresent invention. Basically, a lock-in amplifier is a phase sensitivedetector which can be considered in to include a gain stage 50 includingmatched resistors R1 and R2 (for gain=-1.00), a gain reversing switch 52and an RC filter 54. The position of the switch is determined by thepolarity of a reference input. If the signal input at 56 is a noise freesinusoid and is in phase with the reference signal applied at 58, theoutput of the amplifier 52 will be a full-wave-rectified sinusoidalwaveform. The signal is filtered by the low-pass filter 54. The outputwill be proportional to the value of the output of amplifier 52.However, if the input signal and the reference signal are shifted inphase by 90 degrees, the filtered output of amplifier 52 will be zero.Thus, the output of the integrator will be proportional to the RMS valueof the fundamental component of the input signal, and to the cosine ofthe phase angle between the input signal and the reference. The totaltransfer function of the lock-in amplifier is therefore

    Eout=Ein cosΦ                                          (14)

where Φ is the phase angle between the reference signal and the inputsignal.

Since the gain reversing amplifier and the output filter transfers thesignal information from its input frequency to a DC voltage, the timeconstant of the filter can be made as long as necessary to provide thenarrow band-width required to reject noise accompanying the signal. Thenoise will not add to the output signal since it will tend to causesymmetrical deviations about the true value of the signal in the output.

Voltage-to-frequency converters U11 and U12 are typically voltagecontrolled oscillators, the output frequency of which varies inproportion to the input voltage. Other types of "converters" such asintegrating (dual-slope) analog-to-digital (A-to-D) converters orratiometric A-to-D converters could also be employed for this function.

In a Coriolis mass flow measuring device there can be hydraulic noisepresent in the form of pulsations of the fluid passing through theflowmeter, and there can be unwanted electrical signals picked up by thewiring. The ability of the lock-in amplifier to reject electrical noiseof a mechanical nature that might be present in the pipe in which theflow meter is installed, and to selectively pass through only thosesignals at the same frequency as the reference, and attenuate (by cosΦ)those that are not inphase with the reference, even if they are of thesame frequency, allows the present invention to deal with much higherlevels of vibration and noise than that of a purely digital phasemeasurement system.

Additionally, besides the ability to reject noise introduced into themotion pickups P1 and P2 from electrical, mechanical or hydraulicsources, it is possible that in the design of the motion pick-ups P1 andP2, their harmonic response characteristics may not be preciselymatched. As a result, the harmonic distortion developed by one of thepick-ups can be different from that developed by the other. Thisconsideration is of particular concern in the case where each of the twowaveforms input to the difference amplifier have different harmonicdistortion. Thus, the differences in harmonic distortion between P1 andP2 can provide a substantial contribution to the difference between theP1 and P2 signals.

Because the contribution of the fundamental to the difference signal hasbeen minimized (by gain adjustment of U2 as discussed above), thepresence of harmonic distortion can a create nonlinearity in theresponse of V6 versus mass flow rate for the circuit shown in FIG. 2.However, inclusion of a lock-in amplifier in both the difference channeland the sum channel effectively rejects all of the harmonic distortionpresent, so that linearity is improved from a response standpointwithout having the added expense of designing pick-ups that are linearover their range of motion. Accordingly, the lock-in amplifieraccomplishes many objectives in addition to the rejection ofelectrical/electronic noise introduced from mechanical, hydraulic, orelectrical sources.

In FIG. 5 of the drawing an alternative embodiment of the circuit shownin FIG. 2 is illustrated wherein instead of using the integrator U5 inthe circuit between difference amplifier U3 and lock-in amplifier U7, adifferentiator U5' is used in the circuit connecting the output ofsumming amplifier U4 and the input to lock-in amplifier U8. It will beappreciated that the result of either embodiment is the same.

In FIG. 6, another implementation of the circuit of FIG. 2 is suggestedwherein instead of the voltage-to-frequency and counter circuits U11-U13and U12-U14, analog-to-digital conversion circuits are used to convertthe analog outputs of buffers U9 and U10 into digital signals which canbe processed by the processor.

Another alternative configuration is shown in FIG. 8 wherein the outputsof U9 and U10 are fed into a ratiometric analog-to-digital converter todevelop the output signal to be displayed by the readout device.

FIG. 7 discloses still another alternative configuration for the FIG. 2circuit wherein the output of buffers U9 and U10 are fed into a divider,and the output of the divider is fed into a voltage-to-current orvoltage-to-frequency converter the output of which is then displayed bythe readout.

As pointed out above, the objective of the signal processing method ofthe present invention is to manipulate the signals from the two motionpick-ups P1 and P2 so as to obtain a signal which is proportional toAr/wAe. An alternative way to accomplish this is to divide thedifference signal V3 by the sum signal V4 together with a measurement offrequency w. Because the difference and sum signals are in timequadrature, the reference signal for the lock-in amplifier must beshifted 90 degrees in phase using an integrator or differentiator asshown in FIG. 9. As illustrated, this alternative embodiment providesmeans for dividing the difference signal, which is proportional to N1,by the sum signal, which is proportional to N2, and then divides theresult by a frequency w to obtain a value N1/wN2 which relates linearlyto the mass flow rate.

In FIG. 10 an alternative embodiment of the invention is depictedwherein instead of using the integrator shown in FIG. 2, the voltages V3and V4 are input to a first divider which divides V3 by V4 to develop avoltage V5=V3/V4. This voltage and a frequency value proportion to thedrive frequency value proportion to the drive frequency w are input to asecond divider 62 which develops an output voltage V6=V5/w which isproportional to the mass flow rate. It will of course be appreciatedthat an appropriately programmed microprocessor could be substituted forthe two dividers and develop the voltage V6 in response to the inputsV3, V4 and w.

As discussed above, the approach taught by the Sipin '098, '944, '019patents presents the difference between signals obtained from twovelocity pick-ups to an AC voltmeter and controls the drive amplitude ofthe driving means with the sum of the same two signals obtained frompick-ups. Of the two readily apparent shortcomings of the Sipinapproach, one is indicated by equation (9) above. More specifically, inthe difference voltage V3 it can be seen that there is a frequencyfactor of w introduced in its amplitude. This factor is the frequency ofthe driving voltage, and if there is any variation in that drivingfrequency, such variation will appear as an error in the indicated massflow rate. If the flow meter is operating at resonance, the drivefrequency will naturally vary with fluid density. Thus, in addition tothere being an unwanted dependency on drive frequency, the drivefrequency under resonant operation will vary with fluid density andproduce an unacceptable error.

Conversely, use of the sum signal to control drive frequency presents asecond problem with Sipin's approach in that if there is any aging ofthe components, or if the driving means is replaced, for example, thedrive level will vary, and as is indicated in equation 8 above, if theexcitation amplitude varies, the response amplitude is also going tovary. Accordingly, control of the drive signal based upon a summing ofthe two input signals can result in calibration errors as components ageor are substituted or replaced, or for whatever reason, havecharacteristics which vary. These two shortcomings of the prior art areovercome in the present invention by integrating the difference signalto remove the unwanted frequency dependency. Thus, rather thanattempting to control the influence that amplitude of the driving signalexerts on the mass flow rate measurement based upon the sum of the twosignals, the integrated difference signal is divided by the sum signal.

The above mentioned Dahlin et al '421 patent teaches a tangentdependency of the measured phase difference on mass flow rate. Thisnonlinear relationship is undesirable from the standpoint of one wishingto develop a signal that is linear with mass flow rate, a commonoccurrence because from an electronics engineering standpoint, it iseasier to deal with linear signals than with nonlinear signals. Thepresent invention provides for such a linear relationship.

The teaching of the above mentioned Kappelt '089 patent concerns a phasemeasuring technique for relating phase shift to mass flow rate andindicates that the difference signal approach, using differentialvelocity measurement or differential voltage measurement, is only linearover a small range of phase shifts that may be no more than threedegrees. The present invention likewise overcomes the shortcomingstaught in the Kappelt patent regarding velocity difference methods.

Kappelt also explains how the difference method described in the Smith'450, '338 and '025 patents fails to properly take into accountfrequency, or frequency variations, which can result in mass flowmeasurement errors. Again, the present invention properly takes intoaccount the effect of frequency on the measured signals and removes thatdependency so that there is no problem.

The time difference measurement described in the referenced Smithpatents is also an intermittent type of measurement the accuracy ofwhich can be affected by harmonic distortion in the signals. Smithindicates in his '338 patent, that in order to always "track" theoscillation mid-plane, the motion sensing pick-ups must provide anoutput that is linear over their entire range of movement. Thus, use ofa linear sensor output was required. No such restriction is required inthe present invention and no linearity requirements are made on theresponse characteristics of the pick-ups.

The Smith technique is solely a time based approach that provides anintermittent measurement snap-shot of the condition of the conduit at aparticular point in time wherein other non-Coriolis forces areminimized. Rather than providing a measurement of mass flow rate that isin any way of an intermittent nature, the present invention provides ameasurement that is of a continuous nature throughout the conduit'soscillation cycle rather than an output that is registered onlyintermittently throughout the vibration cycle of the conduit. Thepresent invention is thus a fundamentally different approach in terms ofthe signal processing method itself.

Whereas Smith measures the time difference between the mid-planecrossings of two signals, the present approach determines the electricalpotential difference between two detected signals at the same time andmakes such determination using a difference amplifier. From that pointon, the ability to derive any relative timing information between thetwo input signals P1 and P2 is lost. Furthermore, since the integrateddifference signal and the sum signal are always in phase with eachother, there is no phase measurement of any kind performed between theoutputs of P1 and P2 in the present invention.

The Roth '512 and '257, patents disclose methods for processing analogsignals obtained from "velocity" type pick-ups that respond only toCoriolis induced motion in a circular shaped conduit, and then requiretwo additional pick-ups that respond only to the driving motion in orderto generate a reference signal for the synchronous demodulator used tocompare the signals. In the alternative, a reference signal is derivedfrom the electric motor that is used to excite or drive the conduit. Thepresent invention can be distinguished over Roth in that both themeasurement signals and the reference signal, or signals, are obtainedfrom the same pick-ups.

Roth also discloses the connection of two pick-up coils together inseries opposing fashion. This is unworkable in practice because itrequires close matching of the response characteristics of the sensorsand also requires a conduit structure that behaves in a highlysymmetrical manner.

In FIG. 12 of the Smith '450 patent, an embodiment is illustratedincluding a pair of strain gages feeding a bridge circuit and anamplifier, and a reference signal is fed into a synchronous demodulator.That scheme is part of a force nulling scheme of which the presentinvention is in no way involved. Just like in Roth, Smith providesseparate sensing means, or sensing transducers, and yet additionaltransducers to generate a reference signal. In the present invention themeasurement and reference signals are derived from the same sensors.

Although the present invention has been described above in terms ofvarious alternative embodiments, it will be appreciated by those skilledin the art that additional alternative embodiments, and alterations andmodifications of the illustrated embodiments can be made. It istherefore intended that the appended claims be interpreted as coveringall such alternatives, alterations and modifications as fall within thetrue spirit and scope of the invention.

What is claimed is:
 1. Apparatus for measuring the mass flow rate ofmaterial flowing through at least one vibrating conduit comprising:apair of motion detectors disposed at separate points along the conduitfor detecting motion thereof and for developing first and second motionresponsive analog voltage signals; differencing means responsive to saidfirst and second analog voltage signals and operative to generate adifference signal which is proportional to the voltage differencetherebetween; summing means responsive to said first and second analogvoltage signals and operative to generate a sum signal which isproportional to the sum of the voltages thereof; integrating means forintegrating said difference signal; and dividing means for dividing theintegrated difference signal by said sum signal to develop an outputsignal proportional to the mass flow rate of material flowing throughsaid conduit.
 2. Apparatus for measuring the mass flow rate of materialflowing through at least one vibrating conduit as recited in claim 1 andfurther comprising:comparator means responsive to said sum signal andoperative to develop a reference signal, and a first lock-in amplifiermeans responsive to said integrated difference signal and said referencesignal and operative to cause the integrated difference signal input tosaid dividing means to be a DC signal substantially immune to theeffects of harmonic distortions produced by mechanical, hydraulic orelectrical characteristics of the preceding signal carrying components.3. Apparatus for measuring the mass flow rate of material flowingthrough at least one vibrating conduit as recited in claim 2 and furthercomprising:a second lock-in amplifier means responsive to said sumsignal and said reference signal and operative to cause the sum signalinput to said dividing means to be a DC signal substantially immune tothe effects of harmonic distortions produced by mechanical, hydraulic orelectrical characteristics of the preceding signal carrying components.4. Apparatus for measuring the mass flow rate of material flowingthrough at least one vibrating conduit as recited in claim 3 whereinsaid dividing means includes:a first voltage-to-frequency converter forconverting said integrated difference signal to a corresponding firstalternating signal of a first frequency; a first counter responsive tothe frequency of said first alternating signal and operative to generatea first digital signal proportional thereto; a secondvoltage-to-frequency converter for converting said sum signal to acorresponding second alternating signal of a second frequency; a secondcounter responsive to the frequency of said second alternating signaland operative to generate a second digital signal proportional thereto;and processor means responsive to said first and second digital signalsand operative to develop an output signal which is proportional to saidfirst digital signal divided by said second digital signal.
 5. Apparatusfor measuring the mass flow rate of material flowing through at leastone vibrating conduit as recited in claim 1 wherein said dividing meansincludes:a first analog-to-digital converter for converting saidintegrated difference signal to a corresponding first digital signal; asecond analog-to-digital converter for converting said sum signal to acorresponding second digital signal; and processor means responsive tosaid first and second digital signals and operative to develop an outputsignal which is proportional to said first digital signal divided bysaid second digital signal.
 6. Apparatus for measuring the mass flowrate of material flowing through at least one vibrating conduitcomprising:a pair of motion detectors disposed at separate points alongthe conduit for detecting motion thereof and for developing first andsecond motion responsive analog voltage signals; differencing meansresponsive to said first and second analog signals and operative togenerate a difference signal which is proportional to the voltagedifference therebetween; summing means responsive to said first andsecond analog signals and operative to generate a sum signal which isproportional to the sum of the voltages thereof; integrating means forintegrating said sum signal; first comparator means responsive to theintegrated sum signal and operative to develop a first reference signal;a first lock-in amplifier means responsive to said difference signal andsaid first reference signal and operative to develop a first DC signalwhich is substantially immune to harmonic distortions produced bymechanical, hydraulic or electrical characteristics of the precedingsignal carrying components; second comparator means responsive to theintegrated sum signal and operative to develop a second referencesignal; a second lock-in amplifier means responsive to said sum signalsignal and said second reference signal and operative to develop asecond DC signal which is substantially immune to harmonic distortionsproduced by mechanical, hydraulic or electrical characteristics of thepreceding signal carrying components; and dividing means for dividingsaid first DC signal by said second DC signal to develop an outputsignal proportional to the mass flow rate of material flowing throughsaid conduit.
 7. Apparatus for measuring the mass flow rate of materialflowing through at least one vibrating conduit as recited in claim 1wherein said dividing means includes:a first voltage-to-frequencyconverter for converting said integrated difference signal to acorresponding first alternating signal of a first frequency; a firstcounter responsive to the frequency of said first alternating signal andoperative to generate a first digital signal proportional thereto; asecond voltage-to-frequency converter for converting said sum signal toa corresponding second alternating signal of a second frequency; asecond counter responsive to the frequency of said second alternatingsignal and operative to generate a second digital signal proportionalthereto; and processor means responsive to said first and second digitalsignals and operative to develop an output signal which is proportionalto said first digital signal divided by said second digital signal. 8.Apparatus for measuring the mass flow rate of material flowing throughat least one vibrating conduit as recited in claim 7 wherein saiddividing means includes:a first analog-to-digital converter forconverting said integrated difference signal to a corresponding firstdigital signal; a second analog-to-digital converter for converting saidsum signal to a corresponding second digital signal; and processor meansresponsive to said first and second digital signals and operative todevelop an output signal which is proportional to said first digitalsignal divided by said second digital signal.
 9. Apparatus for measuringthe mass flow rate of material flowing through at least one vibratingconduit comprising:a pair of motion detectors disposed at separatepoints along the conduit for detecting motion thereof and for developingfirst and second motion responsive analog signals; differencing meansresponsive to said first and second analog signals and operative togenerate a difference signal which is proportional to the voltagedifference therebetween; summing means responsive to said first andsecond analog signals and operative to generate a sum signal which isproportional to the sum of the voltages thereof; first dividing meansfor dividing said difference signal by said sum signal to develop acorresponding first quotient signal; means for developing a frequencysignal proportional to the frequency at which said conduit is vibrating;and second dividing means for dividing said first quotient signal bysaid frequency signal to develop a second quotient signal which isproportional to the mass flow rate of material flowing through saidconduit.
 10. A method of measuring the mass flow rate of materialflowing through at least one conduit, comprising the steps of:causing aportion of said conduit to oscillate in displacement relative to a restposition; detecting oscillatory motion of at least two separated pointsalong said portion of the conduit and developing corresponding first andsecond motion responsive analog voltage signals; subtracting said firstsignal from said second signal to develop a difference signal which isproportional to the voltage difference therebetween; adding said firstand second signals to develop a sum signal which is proportional to thesum of the voltages thereof; integrating said difference signal todevelop an integrated difference signal; and dividing the integrateddifference signal by said sum signal to develop an output signalproportional to the mass flow rate of material flowing through saidconduit.
 11. A method as recited in claim 10 and further comprising thestep of synchronously detecting the integrated difference signal priorto division by said sum signal to remove all signal components not inphase with said sum signal.
 12. A method as recited in claim 11 wherethe integrated and synchronously detected difference signal and said sumsignal are digitized and the digitized difference signal is divided bythe digitized sum signal to develop said output signal.
 13. A method asrecited in claim 10 wherein the integrated difference signal and saidsum signals are analog signals and the step of dividing the two signalsis accomplished using an electrical signal processing technique.
 14. Amethod for measuring the mass flow rate of material flowing through atleast one conduit, comprising the steps of:causing a portion of saidconduit to oscillate at a frequency w relative to a rest position;detecting oscillatory motion of the conduit at separate points alongsaid portion of the conduit and developing first and second motionresponsive analog voltage signals; subtracting said first signal fromsaid second signal to develop a difference signal which is proportionalto the voltage difference therebetween; adding said first and secondsignals to develop a sum signal which is proportional to the sum of thevoltages thereof; shifting the phase of a signal proportional to saidsum signal by 90 degrees to develop a reference signal for synchronouslydetecting said difference signal to remove signal components not inphase with said reference signal; dividing the detected differencesignal by said sum signal to develop a corresponding first quotientsignal; developing a frequency signal proportional to the frequency w atwhich said conduit is caused to oscillate; and dividing said firstquotient signal by said frequency signal by said frequency signal todevelop a second quotient signal which is proportional to the mass flowrate of material flowing through said conduit.
 15. A method of measuringthe mass flow rate of material flowing through at least one conduitcomprising the steps of:causing a portion of said conduit to oscillateat a frequency w relative to a rest position; detecting oscillatorymotion of said conduit at separate points along said portion of theconduit and developing first and second motion responsive analog voltagesignals; subtracting said first signal from said second signal todevelop a difference signal which is proportional to the voltagedifference therebetween; adding said first and second signals to developa sum signal which is proportional to the sum of the voltages thereof;dividing said difference signal by said sum signal to develop acorresponding first quotient signal; developing a frequency signalhaving a value proportional to the frequency w at which said conduit iscaused to oscillate; and dividing said first quotient signal by saidfrequency signal to develop a second quotient signal which isproportional to the mass flow rate of material flowing through saidconduit.
 16. A method as recited in claim 15 wherein the integrateddifference signal and said sum signals are analog signals and the stepof dividing the two signals is accomplished using an electrical signalprocessing technique.